1. Field of the Invention
The present invention relates to a Zn-plated or part-Zn plated steel sheet having excellent corrosion resistance and plating adherence, which may be used in car bodies, household electric appliances and buildings. The invention further relates to a method for producing such a plated steel sheet.
2. Description of the Related Art
Zn-plated steel sheets have improved corrosion resistance because of the self-sacrificing corrosion prevention effect of Zn on Fe. This protection against rusting may be enhanced by adding other metals such as Ni, Fe, etc., in the case of part-Zn electroplating, and adding Al, etc., in the case of part-Zn dipping. However, satisfactory corrosion resistance has not yet been obtained.
Recently, Zn--Mg alloy plating has been extensively studied because Mg enhances the rust protection of Zn. Dipping methods employing Mg are disclosed, for example, in Japanese Laid-Open Publications Nos. 56-96036 and 56-123359. However, because the melting point of Mg is much higher than that of Zn (650.degree. C. and 419.degree. C., respectively) and because the eutectic point of Zn and Mg is high, these methods encounter problems in that the amount of Mg which can be added is too small to provide sufficient corrosion resistance. Further, dipping must be performed at such a high temperature that the material properties of the steel sheets are caused to deteriorate;
An electroplating method using Mg is disclosed in Japanese Laid-Open Publication No. 58-144492. Partly because the single electrode potentials of Zn and Mg are far different from each other, this method has not succeeded in providing good plating when the content of Mg exceeds 1 wt %. Thus, the corrosion resistance of the product is not satisfactory.
Evaporation methods are disclosed in Japanese Laid- Open Publications Nos. 64-17851 and 64-17852. Because these methods require high-temperature heat sources and high degrees of vacuum for evaporating Mg, high production costs are inevitable. Also, consistent and even plating layers are hard to obtain by these methods. These problems make it difficult to use these methods on an industrial scale.
Further, Japanese Laid-Open Publication No. 62-109966 discloses a method in which a steel sheet is plated with Zn and the surface of the Zn layer is coated with Mg. Because an evaporation method is used to form the Mg layer, this method also increases production costs. Moreover, the method is confronted with another problem in that the adherence between the Zn and the Mg is not satisfactory.
In summary, as described above, if Mg in metallic form is attempted to be used in Zn plating, many problems are encountered.
Alternatively, Mg may be used in the form of the oxide. Oxide coating, e.g. of SiO.sub.2, MgO, ZrO.sub.2, Al.sub.2 O.sub.3, formed on the surface of a steel sheet or a plated steel sheet reduces the electric conductivity thereof, which slows the corrosion process, and enhances the wear resistance thereof. Corrosion resistance is thus upgraded.
Japanese Laid-Open Publication No. 57-174440 discloses an oxide coating forming method using thermal spraying. However, in products of this method, adherence and workability are not satisfactory. Further, this method tends to produce pinholes.
Other methods employing oxides of Mg are known, such as ion plating as disclosed in Japanese Laid-Open Publication No. 64-65253, or Japanese Laid-Open Publication No. 2-254178 in which a complex coating film of metallic Mg and Mg oxide is formed on top of a Zn plated film by evaporation. However, since these methods require high-degree vacuum equipment and high-temperature heat sources in order to evaporate the materials, high production costs result.
Further, Japanese Laid-Open Publication No. 55-119157 discloses a method in which an oxide coating is formed by applying a water slurry of oxide particles to a steel sheet and then heat-drying the steel sheet. The oxide coating formed in this method is for one-side blocking in zinc dipping and does not function as a protective coating for a steel sheet with favorable workability and adherence. A further method is known in which an oxide coating is formed as a nonconductive coating for a flat rolled magnetic steel sheet by applying coating compositions to the steel sheet and then heat-drying the steel sheet. Again, the oxide coating in this method is not a protective coating of high corrosion resistance, nor is workability of the oxide coating satisfactory.
Japanese Laid-Open Publication No. 1-138389 discloses a Zn--Mg alloy plated steel sheet.
The present invention provides a surface-processed steel sheet having a double plating layer composed of a lower plating layer composed partly of Zn and an upper layer comprising an MgO coating film; the plating layer is formed by cathodic electrolysis in a molten salt containing moisture, oxygen, hydroxide ions and oxygen ions that are actively added.
As described in the prior art, the self-sacrificing corrosion resistance improvement by Zn enhanced by Mg is not satisfactory. Thus, the conventional art has not succeeded in providing a Zn plated or part-Zn plated steel sheet which has good corrosion resistance.